Two new isostructural rare earth phosphates Na₇Mg₁₃<i>Ln</i>(PO₄)₁₂ (<i>Ln</i>=La, Eu) have been synthesized and investigated by X-ray diffraction and optical measurements. They crystallize in the orthorhombic system with the <i>Cmc</i>2₁ space group (<i>Z</i>=4). The crystal structure exhibits a new type of framework built up from <i>Ln</i>O₈ (<i>Ln</i>=La, Eu), <i>M</i>O₆ (<i>M</i>=0.5Mg+0.5Na) and MgO<i>_x</i> (<i>x</i>=5, 6) polyhedra and PO₄ tetrahedra linked by common corner, edge or face. It can be described in terms of [Mg₄MP₄O₂₂]_∞ layers stacked along the <i>a</i> direction. These layers are interconnected by [Mg₄<i>Ln</i>P₄O₃₆]_∞ undulating chains spreading along the <i>b</i> direction. This framework delimits 6 distinct cavities occupied by Na⁺ cations. The results of the optical study of Na₇Mg₁₃La_1−<i>x</i>Eu<i>_x</i>(PO₄)₁₂ (<i>x</i>=0, 0.02, 0.1, 1) reveal the presence of two different Eu³⁺ ion environments whereas the X-ray study predicts the existence of only one Eu site. This difference can be explained by the possible presence of the europium element in the sodium sites with small occupancies which cannot be detected by the X-ray structural determination.< Réduire